The essential function of the immune system is the defense against infection. The humoral immune system combats molecules recognized as non-self, such as pathogens, using immunoglobulins. These immunoglobulins, also called antibodies, are raised specifically against the infectious agent, which acts as an antigen, upon first contact (Roitt, Essential Immunology, Blackwell Scientific Publications, fifth edition, 1984; all references cited herein are incorporated in their entirety by reference). Antibodies are multivalent molecules comprising heavy (H) chains and light (L) chains joined with interchain disulfide bonds. Several isotypes of antibodies are known, including IgG1, IgG2, IgG3, IgG4, IgA, IgD, IgE, and IgM. An IgG contains two heavy and two light chains. Each chain contains constant (C) and variable (V) regions, which can be broken down into domains designated CH1, CH2, CH3, VH, and CL, VL (FIG. 1). Antibody binds to antigen via the variable region domains contained in the Fab portion and, after binding, can interact with molecules and cells of the immune system through the constant domains, mostly through the Fc portion.
B-lymphocytes can produce antibodies in response to exposure to biological substances like bacteria, viruses and their toxic products. Antibodies are generally epitope-specific and bind strongly to substances carrying these epitopes. The hybridoma technique (Kohler and Milstein, 1975) makes use of the ability of B-cells to produce monoclonal antibodies to specific antigens and to subsequently produce these monoclonal antibodies by fusing B-cells from mice exposed to the antigen of interest to immortalized murine plasma cells. This technology resulted in the realization that monoclonal antibodies produced by hybridomas could be used in research, diagnostics and therapies to treat different kinds of diseases like cancer and auto-immune-related disorders.
Because antibodies that are produced in mouse hybridomas can induce strong immune responses in humans, it has been appreciated in the art that antibodies required for successful treatment of humans needed to be less immunogenic or, preferably, non-immunogenic. For this to be done, murine antibodies were first engineered by replacing the murine constant regions with human constant regions (referred to as chimeric antibodies). Subsequently, domains between the complementarity-determining regions (CDRs) in the variable domains, the so-called framework regions, were replaced by their human counterparts (referred to as humanized antibodies). The final stage in this humanization process has been the production of fully human antibodies.
In the art, bispecific antibodies, which have binding specificities for two different antigens, have also been described. These are generally used to target a therapeutic or diagnostic moiety, for instance, T-cell, a cytotoxic trigger molecule, or a chelator that binds a radionuclide, that is recognized by one variable region of the antibody to a cell that is recognized by the other variable region of the antibody, for instance, a tumor cell (for bispecific antibodies, see Segal et al., 2001).
One very useful method known in the art to obtain fully human monoclonal antibodies with desirable binding properties, employs phage display libraries. This is an in vitro, recombinant DNA-based, approach that mimics key features of the humoral immune response (for phage display methods, see, e.g., C. F. Barbas III et al., Phage Display, A laboratory manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 2001). For the construction of phage display libraries, collections of human monoclonal antibody heavy- and light-chain variable region genes are expressed on the surface of bacteriophage particles, usually in single-chain Fv (scFv) or in Fab format. Large libraries of antibody fragment-expressing phages typically contain more than 109 antibody specificities and may be assembled from the immunoglobulin V regions expressed in the B lymphocytes of immunized or non-immunized individuals.
Alternatively, phage display libraries may be constructed from immunoglobulin variable regions that have been partially assembled or rearranged in vitro to introduce additional antibody diversity in the library (semi-synthetic libraries) (De Kruif et al., 1995b). For example, in vitro-assembled variable regions contain stretches of synthetically produced, randomized or partially randomized DNA in those regions of the molecules that are important for antibody specificity.
The genetic information encoding the antibodies identified by phage display can be used for cloning the antibodies in a desired format, for instance, IgG, IgA or IgM, to produce the antibody with recombinant DNA methods (Boel et al., 2000).
An alternative method to provide fully human antibodies uses transgenic mice that comprise genetic material encoding a human immunoglobulin repertoire (Fishwild et al., 1996; Mendez et al., 1997). Such mice can be immunized with a target antigen and the resulting immune response will produce fully human antibodies. The sequences of these antibodies can be used in recombinant production methods.
Production of monoclonal antibodies is routinely performed by use of recombinant expression of the nucleic acid sequences encoding the H and L chains of antibodies in host cells (see, e.g., EP0120694; EP0314161; EP0481790; U.S. Pat. No. 4,816,567; WO 00/63403, the contents of the entirety of each which are incorporated herein by reference).
To date, many different diseases are being treated with either humanized or fully human monoclonal antibodies. Products based on monoclonal antibodies that are currently approved for use in humans include HERCEPTIN™ (trastuzumab, anti-Her2/Neu), REOPRO™ (abciximab, anti-Glycoprotein IIB/IIIA receptor), MYLOTARG™ (gemtuzumab, anti-CD33), RITUXAN™ (Rituximab, anti-CD20), SIMULECT™ (basiliximab, anti-CD25), REMICADE™ (infliximab, anti-TNF), SYNAGIS™ (palivizumab, anti-RSV), ZENAPAX™ (daclizumab, IL2-receptor), and CAMPATH™ (alemtuzumab, anti-CD52). Despite these successes, there is still room for new antibody products and for considerable improvement of existing antibody products.
The use of monoclonal antibodies in cancer treatment has shown that so-called “antigen-loss tumor variants” can arise, making the treatment with the monoclonal antibody less effective. Treatment with the very successful monoclonal antibody RITUXIMAB® (anti-CD20) has, for instance, shown that antigen-loss escape variants can occur, leading to relapse of the lymphoma (Massengale et al., 2002). In the art, the potency of monoclonal antibodies has been increased by fusing them to toxic compounds, such as radionuclides, toxins, cytokines, and the like. Each of these approaches, however, has its limitations, including technological and production problems and/or high toxicity.
Furthermore, it appears that the gain in specificity of monoclonal antibodies compared to traditional undefined polyclonal antibodies, comes at the cost of loss of efficacy. In vivo, antibody responses are polyclonal in nature, i.e., a mixture of antibodies is produced because various B-cells respond to the antigen, resulting in various specificities being present in the polyclonal antibody mixture. Polyclonal antibodies can also be used for therapeutic applications, for instance, for passive vaccination or for active immunotherapy, and currently are usually derived from pooled serum from immunized animals or from humans who recovered from the disease. The pooled serum is purified into the proteinaceous or gamma globulin fraction, so named because it contains predominantly IgG molecules.
Polyclonal antibodies that are currently used for treatment include anti-rhesus polyclonal antibodies, gamma globulin for passive immunization, anti-snake venom polyclonal (CroFab), THYMOGLOBULIN™ for allograft rejection, anti-digoxin to neutralize the heart drug digoxin, and anti-rabies polyclonal antibodies. In currently marketed therapeutic antibodies, an example of the higher efficacy of polyclonal antibodies compared to monoclonal antibodies can be found in the treatment of acute transplant rejection with anti-T-cell antibodies. The monoclonal antibodies on the market (anti-CD25 BASILIXIMAB®) are less efficacious than a rabbit polyclonal antibody against thymocytes (THYMOGLOBULIN™) (press releases dated Mar. 12, Apr. 29, and Aug. 26, 2002, on sangstat.com). The use of pooled human sera, however, potentially bears the risk of infections with viruses such as HIV or hepatitis, with toxins such as lipopolysaccharide, with proteinaceous infectious agents such as prions, and with unknown infectious agents. Furthermore, the supply that is available is limited and insufficient for widespread human treatments. Problems associated with the current application of polyclonal antibodies derived from animal sera in the clinic include a strong immune response of the human immune system against such foreign antibodies. Therefore, such polyclonals are not suitable for repeated treatment or for treatment of individuals that were injected previously with other serum preparations from the same animal species.
The art describes the idea of the generation of animals with a human immunoglobulin repertoire, which can subsequently be used for immunization with an antigen to obtain polyclonal antibodies against this antigen from the transgenic animals (WO 01/19394, the entirety of which is incorporated herein by reference). However, many technological hurdles still will have to be overcome before such a system is a practical reality in larger animals than mice and it will take years of development before such systems can provide the polyclonal antibodies in a safe and consistent manner in sufficient quantities. Moreover, antibodies produced from pooled sera, whether being from human or animal origin, will always comprise a high amount of unrelated and undesired specificities, as only a small percentage of the antibodies present in a given serum will be directed against the antigen used for immunization. It is, for instance, known that in normal, i.e., non-transgenic, animals, about 1% to 10% of the circulating immunoglobulin fraction is directed against the antigen used for hyper-immunization; hence, the vast majority of circulating immunoglobulins is not specific.
One approach towards expression of polyclonal antibody libraries has been described (WO 95/20401; U.S. Pat. Nos. 5,789,208 and 6,335,163, the contents of the entirety of each of which are incorporated herein by reference). A polyconal library of Fab antibody fragments is expressed using a phage display vector and selected for reactivity towards an antigen. To obtain a sub-library of intact polyconal antibodies, the selected heavy and light chain-variable region gene combinations are transferred en mass as linked pairs to a eukaryotic-expression vector that provides constant region genes. Upon transfection of this sub-library into myeloma cells, stable clones produce monoclonal antibodies that can be mixed to obtain a polyclonal antibody mixture. While in theory it would be possible to obtain polyclonal antibodies directly from a single recombinant production process using this method by culturing a mixed population of transfected cells, potential problems would occur concerning the stability of the mixed cell population and, hence, the consistency of the produced polyclonal antibody mixture. The control of a whole population of different cells in a pharmaceutically acceptable large-scale process (i.e., industrial) is a daunting task. It would seem that characteristics, such as growth rates of the cells and production rates of the antibodies, should remain stable for all of the individual clones of the non-clonal population in order to keep the ratio of antibodies in the polyclonal antibody mixture more or less constant.